[admiralpye]

Drills with lots of explosive movement involved. Try plyometrics and lunges. Tennis.com has a good collection of tennis drills and exercises you can do. Push ups or pull ups and planks are great too because it can help strengthen arms and chest for serves and overheads. Don't neglect your abs. You have more stamina if your abs are stronger. Running will target ab strength somewhat. But do lots of sprinting instead of long-distance running. You'll have more use for short, fast bursts of running with lots of starts and stops than marathon running.

Good luck!